The field of this invention relates to offshore rigs, particularly the type adapted to be mounted on an offshore platform.
So far as known, it is typical for a single rig to be movably mounted on skids on the offshore platform to allow the rig to be movably disposed about the center line of numerous wells that are typically serviced by the off-shore platform. Such a rig typically performs many different types of functions about the offshore platform. These operations may include performing functions as a service rig, providing for downhole pump maintenance, being used for initial well completion as well as for recompletion of such wells, for casing tie-backs from subsea wellheads, and for drilling, running and cementing of tubular members. In most instances, these operations occur at relatively shallow depths with respect to the mud line of the ocean floor which supports the offshore platform. However, to perform these functions, it is heretofore been necessary that the main rig be tied up performing these functions, which, because of the significant costs involved in using such a rig, typically run on the order of $15,000.00 per day.
Typical activities such as downhole pump maintenance are complicated by the fact that an electricalhydraulic conductor attached to a tubular member and/or an electrical conductor-cable must be reeled in along with the removal of the pump from the downhole position. In most instances, such operations all take place on the deck of the rig requiring multiple people and operations to be occurring simultaneously at one area which not only hinders the speed at which such may be removed from the well, but also increases the potential of safety hazards.
Also, because of the tremendous weights involved, it has been found that upon making connection of the service riser with the rig that significant compressive loading on the service riser may result. This compressive loading in addition to the normal stresses and strains encountered by the riser may lead to premature failure of its component parts, which could result in costly time delays.
Still further, in most applications a mousehole is used for storing such tubular members as drilling pipe prior to their actually being used in drilling operations of the rig. However, once the pipe that is in the mousehole is used, the mousehole must then be resupplied for further use. This procedure thus results in a time-consuming operation which, because of the large costs involved, can be significant.
Furthermore, so far as known, no structure for providing lifting capabilities for an offshore rig has been mounted with the upper end of a racking mechanism to permit vertical lifting and straight-line movement of such elevated tubular members, such as casing, as opposed to angularly disposed or otherwise positioned lifting and/or rotational movement thereafter of such tubular members. To provide for such a vertical raising and lowering of tubular members, then a requirement exists that the center line of the lifting mechanism be able to move from a position where it is aligned over the object to be lifted, thereafter being capable of lifting the same, and disposing it in such a manner that it is substantially aligned with the center line of the well. So far as known, no such structure has been employed in the field of offshore rig operations on an offshore platform.